A known problem in this field is that the bolts of locks can be subjected to heavy forces in an attempt to force or break open a locked door. These forces may be generated by heavy blows with a hammer-like tool or may be generated with the aid of a crowbar or like tool. The object is to destroy the latching function of the lock, so as to enable the bolt or latch to be moved to an inward, non-locking position and therewith allow the door to be opened. Because the interlocking pin or like member takes-up a large part of the force exerted, the pin must be dimensioned to absorb these forces, which is difficult to achieve. Otherwise, a weak lock construction must be accepted.
The European Patent Specification EP-A1-0 290 348 describes a lock bolt which can be blocked in an outwardly extended, locking position when subjected to violence from outside the lock. The lock mechanism includes a moveable element which in an outwardly extended position engages a weakened region in the bolt and therewith causes permanent deformation of the bolt when subjected to heavy forces from outside the lock. The bolt is not in itself self-locking, and the moveable element is required in order to achieve permanent locking of the lock.